Abstract

The Upper Ordovician sediments of the Bohdalec (Katian) and lower layers of the Králův Dvůr (Katian) formations were exposed at Dolní Počernice, a suburb in the eastern part of Prague, during driving of an underground conduit along the Nad Rybníkem Street at the turn of 2017 to 2018. The full sequence of sediments was not accessible and the investigation was restricted to the material deposited on a nearby dump where it was concentrated mainly on the dark grey siltstone and shale of the Bohdalec Formation, which allowed detailed study of this formation exposed only at a few lasting outcrops. Large amount of these rocks (about 3.000 kg) were transported to the storage place of National Museum at Horní Počernice during several months, and subsequently studied in detail. These occasional outcrops provided a poorly diversified assemblage of invertebrates, characterized by dominance of graptolites and trilobites. This paper focuses on the most common trilobite species Onnia superba (Bancroft, 1929). The studied taxon occurred here mainly as meraspid shields, both cephalic and pygidial or as completely preserved (mostly juvenile) specimens. Several hundreds of collected trilobites and their accumulations document almost a complete ontogenetic development from the degree M1 up to the late holaspid specimens (no protaspid specimens were found). Some meraspid trilobites show marks of disarticulation or typical exuviation patterns, and they can be considered as disintegrated exuviae. On the other hand, outstretched or enrolled complete meraspid specimens or their accumulations (regarded as carcasses) were preserved with articulated free cheeks including long genal spines. These specimens and accumulations provide valuable taphonomical and paleoecological information, whose additional interpretations (together with the study of Onnia superba ontogeny) will be a subject of next investigation.